Current mode output driver circuits or differential current amplifier circuits are well known in the art. They are preferred over voltage mode output driver circuits because the slew rate of the output signals can be easily controlled. In addition, they can directly drive a pulse transformer. Typical applications include use in ethernet transceivers.
In the past, to implement a current mode driver circuit to drive a 50 ohm resistor, with one terminal connected to ground, the driver circuits were comprised of either PMOS transistors or high frequency PNP transistors. One example of a prior art current mode driver circuit 10 is shown in FIG. 1.
In FIG. 1, the driver circuit 10 comprises two vertical type PNP transistors 18 and 20 for receiving the input signals 26 and 28 at the bases thereof, respectively. The collectors of the PNP transistors 18 and 20 are respectively supplied to two 50 ohm resistors 22 and 24 and connected to ground. The collectors of the two PNP transistors 18 and 20 supply the outputs 30 and 32 of the output mode driver circuit 10, respectively.
The emitters of the PNP transistors 18 and 20 are connected to a single PNP transistor 16, preferably also vertical type, at its collector. The emitter of the PNP transistor 16 is connected to a voltage source such as Vcc. The base of the PNP transistor 16 is connected to the base of a matched PNP transistor 12 also at its base. The emitter of the PNP transistor 12 is also connected to the same voltage source Vcc. The base and the collector of the PNP transistor 12 are connected together and are connected to a current source 14 which is connected to ground. The PNP transistors 12 and 16 act as a current mirror.
The foregoing description of the prior art current mode driver circuit 10 consisting of PNP transistors 12, 16, 18, and 20 could also be replaced by PMOS FET transistors.
The shortcoming of the current mode driver circuit 10 of the prior art is that the circuit is comprised of either PNP transistors or PMOS FETs. Because the current mode driver circuit 10 has to operate under fast switching conditions, the process to make the fast switching PNP transistors, such as vertical type, or PMOS FET transistors, having high current density, is costly.